This invention concerns control of the volume rate of flow of air through a duct. More particularly it concerns a control for maintaining a constant volume rate of flow of air through the duct at a predetermined sensed condition and resetting the constant volume rate of flow of air through the duct as functions of two sensed conditions.
In U.S. Pat. No. 4,042,173, issued Aug. 16, 1977, George C. Boyer and Raymond J. Fermanich disclose a control for maintaining a constant volume rate of flow of air through a duct at a predetermined sensed condition and resetting the constant volume rate of flow of air through the duct as a function of the sensed condition. The present invention is an improvement upon a portion of that patented control. The dual reset controller, which is the subject of this invention is independent of the employment or non-employment of the aerodynamic bias covered in the patent.
Roger P. Engelke and Marvin Zille in U.S. Pat. No. 3,809,314, issued May 7, 1974, disclosed a similar, but self-powered controller.
The apparatus in these patents and all devices of which the inventor is aware for resetting constant volume rates of flow of air through a duct are reset in response to a single sensed condition, which is all that is required in many applications. In other applications, such as those requiring morning warm-up or reheat, additional controls have been required to control these functions. This is because a certain flow of air is required to distribute the heat provided during warm-up or reheat by heat exchangers located in the duct, and, when electric heat is supplied, to prevent burn-out of the heating coils. The additional controls are expensive and increased the volume of control air required for their operation. The larger volume of required air in turn must be supplied by larger compressors and be delivered through larger tubes, both of which increase initial cost. Furthermore, the larger compressor requires more energy for its operation, thus increasing the operating cost of the system.